International Patent Application No. WO 02/09966 discloses a transmission system for use in a four wheel drive vehicle for transferring the torque between the front and rear axles in accordance with requirements. The transmission system disclosed therein includes an epicyclic differential mechanism, the annulus of which constitutes the input of the transmission and the carrier and sun wheel of which constitute its two outputs which are connected, in use, to respective pairs of driven wheels of the vehicle. The two output shafts carry first and second coaxial sun wheels of an epicyclic gear system which mesh with first and second sets of planet wheels, respectively. The epicyclic gear system includes a third sun wheel, which is coaxial with the first and second sun wheels and is in mesh with a third set of planet wheels. The gear ratios of the first sun wheel with the first set of planet wheels, the second sun wheel with the second set of planet wheels and the third sun wheel with the third set of planet wheels are different. Each first planet wheel is connected to respective second and third planet wheels to rotate therewith about a respective common planet shaft. The planet shafts are connected to a common carrier which is rotatably mounted coaxially with the sun wheels. The carrier is connected to a first selectively operable brake and the third sun wheel is connected to a second selectively operable brake. The transmission system includes a sensor arranged to produce a signal indicative of an operating parameter of the vehicle, such as its acceleration, and a controller connected to the sensor and to the two brakes and arranged to operate the brakes in response to the said signal. Operation of the brake connected to the third sun wheel produces a torque transfer from one output shaft to the other and operation of the other brake produces the reverse effect.
British Patent Application No. 0203026.0 discloses a somewhat modified transmission system in which the third sun wheel and associated set of planet wheels are omitted and single speed changing device, e.g. an electric motor, is connected to the common carrier and arranged to speed it up or slow it down. Thus, it was realised that speeding up the carrier has the same effect as slowing down the third sun wheel and thus that the third sun wheel is not needed at all.
Both of the above transmission systems are extremely effective but do suffer from one problem. Thus each planet wheel is connected to one or two further planet wheels with which it forms a rigid unit. It is found in practice that it is extremely difficult to align the teeth on the planet gears relative to one another and to repeat this for each group of planet wheels. This means in practice that all the torque is transmitted through only one of the sets of planet wheels and that the other sets transmit little or no torque at all. If the misalignment error is greater than the backlash between the gear teeth it is impossible to assemble the system. This means that one of the primary advantages of epicyclic gearsets, namely the splitting of the torque transmission between, typically, three or four planet wheels is lost. Since one can not readily predict which set of planet wheels will be the one that transmits the torque, the practical solution is to provide only one set of planet wheels. However, since this set will necessarily transmit all the torque, it will have to be very much more massive than would otherwise have been the case and thus results in an unacceptable increase in size of the transmission system.